This invention relates to captive fasteners for press-in installation into panels or the like.
Captive fasteners are mechanically attached to a mating component or panel, such as a releasable access panel, a face plate, or an electronic chassis, and are retained to the mating component so as to prevent loss during use. Common captive fasteners include a screw element having a shank with a threaded portion, such as a threaded stud, and an attaching collar referred to as a ferrule assembled with the screw element. When the threaded portion is unfastened from the panel, the ferrule restrains the captive fastener against disengagement from the panel. Captive fasteners ease assembly and speed disassembly without the occurrence of loose fastening components.
One conventional method for attaching the captive fastener to the panel or mating component is to press-in the ferrule into the panel. To that end, the material forming the ferrule must be harder and less malleable than the material forming the panel to achieve a constant flow of panel material around the ferrule""s mounting profile during press-in installation. The engagement between the panel material and the ferrule creates an obstruction that positively captivates the ferrule with the panel. Serrations, rings, barbs or protrusions are often added to the ferrule profile to create interference and displace the softer, more malleable panel material. Other conventional methods for achieving a press-in engagement between the ferrule and the panel include a multi-profile design with a flange at the end of the ferrule, a groove located next to the flange at the flanged end of the ferrule, or a protrusion element such as a ring or knurl pressed and embedded into the material of the mating component during installation. In all press-in methods, the ferrule material must be harder than the panel material to allow the panel material to flow to create a reliable and secure engagement between the ferrule and the panel.
One familiar type of press-in captive fastener is assembled by inserting the threaded portion of the shank of the screw element through a bore in the ferrule. Subsequently, a rim or edge of the ferrule is bent down or crimped to close the bore and create a smaller inner diameter around an unthreaded smaller diameter portion of the shank of the screw element. The rim may be located at an upper edge of the ferrule, at a mid-section or higher as long as the rim is fastened above the threads of the fastener to capture the threaded region. This smaller diameter opening in the ferrule prevents the diametrically larger threaded portion of the screw element from passing back through the ferrule so that the ferrule is captured to the screw element.
One major disadvantage with this conventional press-in captive fastener design is that the ferrule must be soft and malleable to allow it to be crimped and bent around the shank of the screw element. The need for a malleable or ductile ferrule places serious restrictions on the use of this type of captive fastener. Specifically, conventional captive fasteners of this type may only be installed into panels or other surfaces that are softer than the ferrule, typically a surface hardness less than about 60 on the Rockwell B scale (Rockwell B60). As a result, conventional press-in fasteners of this design are not usable with panels formed of most popular carbon steels having hardness greater than Rockwell B60.
Therefore, there is a need for a press-in captive fastener for use with panels having a surface hardness greater than that of the panel and in which a portion of the ferrule is captured with the screw element by deformation.
The invention relates to press-in captive fasteners for use with panels in which a portion of the ferrule is crimpable for capturing the screw element. According to one embodiment of the invention, a captive fastener assembly is provided for press-in installation into an opening formed in a panel. The captive fastener assembly includes a screw element having a head and a shank projecting from the head and a ferrule having a central bore receiving the shank of the screw element. The shank of the screw element includes a threaded portion and an unthreaded portion disposed between the head and the threaded portion. The ferrule has an upper rim deformed radially inward to limit movement of the ferrule relative to the screw element by interference with the threaded portion. The ferrule also has a penetration tip having a surface hardness greater than the upper rim and/or the panel for press-in installation into the opening in the panel.
In accordance with the principles of the invention, the gradient hardening of the ferrule permits the captive fastener to be used in panels formed of a material harder than the upper rim, typically over Rockwell B60 hardness, and still provide a simple and effective way of assembling the ferrule to the screw element. The ability to crimp a portion of the ferrule to couple the screw element with the ferrule simplifies the construction of the captive fastener in that an additional retainer or locking element is not required for coupling the ferrule with the screw element.